a wealth of good information here.
just hoping someone will have better knowledge than me. A little side track, not DLC but still download content related.
What about China/Hong Kong?
i can confirm that there are 2 different PSN stores for these 2 regions, even tho they are the same country.
reason im asking is that i was tempted to buy some game download codes from china, some of them are relatively cheaper, but the seller will always state that u can only use a HK/China account, no vice versa. Im wondering if the HK codes can be used for our Sg account?
Within Asia, each country has separate PSN store with variation in contents. Like HK and TW will have lots of Traditional Chinese games and also Japanese language games. For whatever stupid reason, many such Traditional Chinese and Japanese games are not available on South East Asia (SG, MY etc) psn stores. (Incredibly, the English versions of those same games are on SEA stores but not HK/TW, go figure.) China store is very very empty, and their games are Simplified Chinese. You practically don't see Simplified Chinese games anywhere else, even SG (though there are a few games with multiple languages that includes SimCh).
I am not sure about download codes though, cos I have never gotten any code to try. If the download code is wallet top-up card, then it definitely will not work. These pre-paid card are for specific currency (Chinese Yuan, Hong Kong Dollar, SGD, etc) and PSN store does not do currency exchange conversion. So if you get China top-up card, you can only use it on China psn store, and with that wallet credit you can only buy things from China psn store (which doesn't have much stuff). If the download code is for certain game/DLC, then obviously it won't work if the item is not available in the country you want to redeem. If the code is for an item common to multiple countries, that's the part I am not sure as I have not tested. There is a possibility that it
may work, just like a Canada code can work in US (they are both the US region).
Regardless, you can just create an account in the same country as the code, then download and use another country's account to play it. This is not 100% though (eg. wallet top-up, games not available in another country, DLC and game incompatible across countries, certain game/DLC are locked to account that purchased it, ...).
I doubt a China code can be redeemed by a HK account (or any other Asia country account) due to it being a very different entity entirely. You can still create China account to redeem China code, but the question is whether the item you redeemed is useful to your other accounts. If the code is for a particular game which you don't mind playing in Simplified Chinese, then perhaps ok. If it's for PS+ subscription, then remember that they have very little content on China store and give very few free games, but the PS+ subscription benefit of getting online access should be sharable with other accounts.
I have left out Korea (South Korea) in this, as you cannot create a Korea account. Korea government has some sort of online identity thing, whereby each person has a single account (using real personal info) to access many online services. So you cannot create Korea psn account unless you are a Korean or staying there and have the online identity credential.